The proper function of organisms, their organs, and their tissues requires them to have specific shape. How this shape is specified and maintained is a fundamental question in biology. In animals, the shape is determined through the process of morphogenesis, a concerted sequence of tissue remodeling events leading up to the final body plan. Despite a long-standing effort to understand the physical mechanisms that underlie morphogenesis, these mechanisms still remain largely unknown. In the past, a large number of molecular players involved in regulating morphogenesis have been identified and characterized in great detail. In contrast, physical mechanisms through which the various molecular players collectively determine the tissue dynamics remain poorly understood. Basic considerations from physics imply that in order to completely determine the mechanism of a morphogenetic change, two pieces of information are absolutely required: (1) the knowledge of active forces that drive tissue deformation, and (2) the knowledge of material properties of the tissue. Using Drosophila gastrulation as a model, and by combining biophysical, molecular, and modeling methods, we propose an approach sufficient to determine both. In this way, we anticipate being able to for the first time completely characterize the physical principles that underlie one of the most studied morphogenetic events. Both the techniques and general approach developed here should be applicable to a wide variety of tissue morphogenesis processes.